For adapters configured by [[netcfg]], another way to set the mtu persistently is to insert a line like this in the network profile:

For adapters configured by [[netcfg]], another way to set the mtu persistently is to insert a line like this in the network profile:

Revision as of 21:12, 16 June 2013

From Wikipedia: In computer networking, jumbo frames are Ethernet frames with more than 1,500 bytes of payload (MTU). Conventionally, jumbo frames can carry up to 9,000 bytes of payload, but variations exist and some care must be taken when using the term. Many, but not all, Gigabit Ethernet switches and Gigabit Ethernet network interface cards support jumbo frames, but all Fast Ethernet switches and Fast Ethernet network interface cards support only standard-sized frames.

Examples

It is important to test various frame sizes doing typical file transfers to determine the optimal setting. There is not a one-size-fits-all setting for jumbo frames. In fact, as illustrated below, depending on the hardware, a frame size that is too large can actually hurt network throughput.

Three different frame sizes (standard 1500, jumbo 4000, and jumbo 9000) were tested under several different conditions (single large file and multiple small files). The results indicate that for this particular hardware, a 4000 frame size gave the optimal results. Again, this result is not general; users will need to evaluate the effect that different frame sizes have on different file transfers on a case-by-case basis.

Both machine A and machine B are using approx. 5-year-old EIDE hardrives; a modern SATA II HDD and MB will give higher throughput for sure, but the data are still valid. Also, both machines are using Dlink 530T NICs (PCI bus) and are set to the frame size indicated in the tables below (1500, 4000, and 9000).

Test 1 - Single large file (1,048,522 kb) via Samba

Times and throughput represent the average of three runs

mtu=1500

t (sec)

Kb/sec

mtu=4000

t (sec)

Kb/sec

mtu=9000

t (sec)

Kb/sec

A to B

48

21,844

A to B

44

23,830

A to B

49

21,398

B to A

81

12,945

B to A

41

25,574

B to A

41

25,574

Summary of Test 1

4k vs. 1.5k

% change

9k vs. 1.5k

% change

A to B

+9 %

A to B

-2 %

B to A

+98 %

B to A

+98 %

Test 2 - Several small files (1,283,439 kb total) via Samba

Times and throughput represent the average of three runs

mtu=1500

t (sec)

Kb/sec

mtu=4000

t (sec)

Kb/sec

mtu=9000

t (sec)

Kb/sec

A to B

59

21,753

A to B

51

25,165

A to B

57

22,516

B to A

94

13,654

B to A

46

27,901

B to A

49

26,193

Summary of Test 2

4k vs. 1.5k

% change

9k vs. 1.5k

% change

A to B

+16 %

A to B

+4 %

B to A

+4%

B to A

+92 %

Sustained Transfer Speed

An average speed of 26.6 MB/s on a 30+ GB xfer using 4k jumbo frames. The slowest drive in the link was a Western Digital WD3200JB - 5-year-old, EIDE 320 GB (ATA100/7200 RPM/8 MB cache). Burst speeds (i.e. 300-400 meg files) through the network are about 80% of the burst speed drive-to-drive which is not too bad (approx. 30 MB/s network vs. approx 38 MB/s drive-to-drive).

Example LAN Architecture using Jumbo Frames

In the above example, all the PCs have NICs set to use JFs and are all connected to a GigLAN switch that can also use JFs. The switch is in turn connect via the uplink port to a router that cannot use jumbo frames which is in turn connected to a cable modem which also cannot use jumbo frames.

Contrary to what some websites state, this setup works 100 % fine. Xfers inside the JF portion on the network (i.e. behind the switch) are very fast. Xfers to the WAN (the Internet) from PCs behind the switch are just as fast as a PC without JFs enabled connected either directly to the cable modem, or to the router.